Improve the error message when redefining a builtin function

[developedby]

Is your feature request related to a problem? Please describe.
When redefining a builtin function, the error message just shows Redefinition of function 'XXX'. This is a bit confusing if the user doesn't know that XXX is a builtin, since it seems to be coming out of nowhere and doesn't point to the first definition.

Describe the solution you'd like
It would be better if the message showed Redefinition of builtin (function) 'XXX'.

Describe alternatives you've considered
Storing the span of functions and printing the context of the first definition. This would greatly increase the complexity of the parser and slightly increase the complexity of the AST, so I"d rather not do it unless we really need to.